The present invention relates generally to distributed amplifiers, and more particularly relates to techniques for increasing an overall gain in a distributed amplifier without proportionally increasing a quiescent current in the distributed amplifier.
Distributed amplifiers are well-known in the art. Such distributed amplifiers generally include a plurality of basic amplifying stages that are connected between input and output transmission lines such that the outputs of the basic amplifying stages are combined to produce a resultant amplified signal.
Each of the basic amplifying stages in a distributed amplifier contain reactances, mostly capacitive, which affect the input and output impedances of those stages. A properly designed distributed amplifier compensates for these reactances so as to minimize the effect upon the transfer of power in a desired frequency range of operation. Conventionally, this has been accomplished by including compensation networks coupled to the input and output transmission lines. These compensation networks typically include both inductances and capacitances along their lengths so as to appear as short lengths of transmission lines having specific characteristic impedances.
The input capacitance of each amplifying stage when separated by an inductance, whether lumped or distributed, determines the characteristic impedance of the overall transmission line. One problem with conventional distributed amplifier architectures is that as a signal propagates down the input transmission line it is attenuated, and the power input to each successive amplifying stage therefore becomes significantly reduced. This problem exists on the output transmission line as well, although it is generally less significant because the signals are amplified by the transconductance of each amplifying stage. For a given number of amplifying stages, the decay in input power degrades the total output power and gain available from the distributed amplifier.
Conventional techniques for reducing the input power decay include coupling additional amplifying stages to the distributed amplifier. However, because of the decaying input signal, there is a point of diminishing returns to this approach. Moreover, the use of additional amplifying stages increases both the size and current consumption of the distributed amplifier. Another conventional approach for increasing the overall gain of the distributed amplifier is to uniformly increase the transconductance of each of the amplifying stages to account for worst case loss on the transmission line. However, since uniformly increasing the transconductance of the amplifying stages generally involves increasing the size of one or more transistors in each of the amplifying stages of the distributed amplifier, this approach can significantly degrade the frequency response and undesirably increase the quiescent current in the distributed amplifier.
The present invention provides techniques for efficiently improving an output transimpedance and/or increasing an overall gain in a distributed amplifier without proportionally increasing the quiescent current and/or noticeably degrading the frequency response in the distributed amplifier. Moreover, such performance enhancements are achieved without a significant increase in the size of the distributed amplifier.
In accordance with one aspect of the invention, a distributed amplifier having an improved output transimpedance includes a plurality of amplifying stages operatively coupled between an input transmission line and an output transmission line. Each of at least a subset of the amplifying stages has a transconductance associated therewith which is operatively configured so as to produce a gain in the amplifying stage which substantially compensates for an input signal attenuation on the input transmission line and/or output transmission line. Thus, the transconductance of each successive amplifying stage in the subset is increased in accordance with the input line loss at the input of the respective amplifying stage. In this manner, each amplifying stage in the subset is individually configured to compensate for an input transmission line loss. In a preferred embodiment of the present invention, every amplifying stage in the distributed amplifier is configured so as to have a transconductance which substantially compensates for an input signal attenuation at a respective input and/or output on the input transmission line and/or output transmission line.
In accordance with another aspect of the present invention, a method is provided for forming a distributed amplifier including a plurality of amplifying stages. Each of the amplifying stages has a transconductance associated therewith and includes an input and an output, the inputs of the plurality of amplifying stages being operatively coupled to an input transmission line and the outputs of the amplifying stages being operatively coupled to an output transmission line. The method comprises the steps of: determining an input signal attenuation at the respective inputs of at least a subset of the plurality of amplifying stages; and selecting the transconductances of each of the amplifying stages in the subset such that a gain of each of the plurality of amplifying stages in the subset substantially compensates for an input signal attenuation on the input transmission line at the respective inputs of the amplifying stages.